This invention relates to communication systems; and more particularly, it relates to point-to-multipoint CDMA communication systems.
As used herein, the term "point-to-multipoint" refers to a communication system in which a single transmitting station that is located at one particular point sends separate data sequences to multiple receiving stations which are located at various other points. That is, a first data sequence D.sub.1 is sent to a first receiving station, a second data sequence D.sub.2 is sent to a second receiving station, etc.; and, all these data sequences are sent at the same time.
One way to operate such a system is to have the transmitting station send each data sequence as an amplitude modulated or frequency modulated or phase modulated signal in its own wireless channel which differs in frequency for each receiving station. However, if the total number of receiving stations in the communication system is large, then a corresponding large number of separate frequency bands is required. Alternatively, the transmitting station can send each of the data sequences over a separate cable to the respective receiving stations. However, when the receiving stations are remotely located from the transmitting station, too much connecting cable is required.
By comparison, with a point-to-multipoint CDMA communication system, the transmitting station sends all of the data sequences in either a single wireless channel or a single cable. By the term "CDMA" is herein meant "Code Division Multiple Access". In a CDMA system, the transmitting station encodes each data sequence that it sends with a respective spreading code which is unique to the receiving station which is to receive the data sequence. That encoded data, for all the receiving stations, is sent simultaneously on a single wireless channel/cable in one frequency band to all of the receiving stations. Then, in each receiving station, the data in any one particular sequence is recovered by multiplying the composite CDMA signal by the same spreading code which was used in the transmitting station to encode the data sequence.
One prior art CDMA communication system is described in U.S. Pat. No. 4,908,836 by Rushforth, et al, entitled "Method and Apparatus for Decoding Multiple Bit Sequences That Are Transmitted Simultaneously in a Single Channel". Also, another CDMA communication system is described in U.S. Pat. No. 5,031,173 by Short, et al, entitled "Decoder for Added Asynchronous Bit Sequences". Both of these patents are assigned to the assignee of the present invention.
In the prior art, the transmitting station of the CDMA communication system combined all of the encoded data sequences that were sent simultaneously with analog circuitry; and this circuitry included a separate IF stage for each concurrent data sequence that was transmitted. Consequently, such a transmitter requires a large amount of circuitry when the number of concurrently transmitted data sequences is large.
Also, in the prior art, the composite signal from the transmitting station has a peak-to-average power ratio which increases as the number of concurrently transmitted data sequences increases. This is a problem when the transmissions occur on a channel which has a peak power constraint, since it means that the average power in the transmitted signal decreases as the number of concurrent data sequences increases. And, as the average power decreases, the maximum distance over which the signal can be received decreases.
Accordingly, a primary object of the present invention is to provide an improved point-to-multipoint communication system in which the above-drawbacks are overcome.